Understanding the Catalytic Site in RuOx-TiOx Electrodes for Chlorine Evolution Reaction

Abstract

As late as in 2017, there were still at least one third of world population suffering from water sanitation problems. Electrochemical oxidation (EO) technology, which relies on anodic generation of reactive chlorine and oxygen species as oxidants and disinfectants, has been shown feasible for human wastewater onsite treatment and recycling. Pivotal to the EO system is anodic chlorine evolution reaction (CER). Traditionally, oxides of platinum group metals (PGMs) have shown excellent catalytic activity for CER, with RuO2 identified as the golden standard. However, fast acid dissolution and prohibitive price prevents its real application. On the other hand, RuOx-TiOx binary electrodes (denoted as RTO electrodes) have demonstrated both enhanced activity and stability compared to pure RuO2, and it has been used in chloro-alkali industry for centuries as one of the dimensionally stable anodes (DSA). Previous research has ascribed the exceptional activity of RTO electrodes to Ti-Ru polarizations pairs (denoted as RuOx \U0001d6ff +-TiOx \U0001d6ff -). Nevertheless, the actual catalytic site (RuOx \U0001d6ff + or TiOx \U0001d6ff -), as well as the specific mechanism, has remained elusive.In this work, ultrathin sandwich electrodes were fabricated by sequentially depositing nanometer-thick uniform crystalline TiO2 and RuO2 films onto HF-treated atomically flat Si wafers to study the active sites in RTO electrodes. The electrodes were characterized with scanning electron microscope (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM), while the CER catalytic activity was probed by chronopotentiometry experiments in 5 M NaCl solution. The structure of the sandwich electrodes was varied to investigate the activity of both components, and layer thickness was varied to quantify the effective range of Ti-Ru polarizations. Density functional theory (DFT) computation was used to help understand the catalytic mechanism.In order to promote further application of EO systems in developing countries, electrodes at lower price are essential. Not only does this work shed light on the catalytic mechanism of RTO electrodes, it also provides important guidance for design of low-PGM or PGM-free electrodes towards high-efficiency CER catalysis and wastewater treatment.